My mira shower will not heat. Stays constantly on cold?

Mira advance thermostatic power shower does not heat. Fitted new hose on shower two days ago, as old hose had split. Noticed after fitting hose some leakage from where shower hose links to main box. Yesterday evening while taking shower noticed that shower stayed constantly on cold and would not switch off, unless from main pull switch. Currently do not have any hot water. Please can anyone tell us what has happened and cause of problem, and advise us on how to remedy it. Many thanks!